Early in Go 1.5 we had bugs with ownership of workbufs, so we added a
system for tracing their ownership to help debug these issues.
However, this system has both CPU and space overhead even when
disabled, it clutters up the workbuf API, the higher level gcWork
abstraction makes it very difficult to mess up the ownership of
workbufs in practice, and the tracing hasn't been enabled or needed
since 5b66e5d0 nine months ago. Hence, remove it.

Benchmarks show the usual noise from changes at this level, but little
overall movement.

name              old time/op  new time/op  delta
XBenchGarbage-12  2.48ms ± 1%  2.47ms ± 0%  -0.68%  (p=0.000 n=21+21)

name                      old time/op    new time/op    delta
BinaryTree17-12              2.98s ± 7%     2.98s ± 6%    ~     (p=0.799 n=20+20)
Fannkuch11-12                2.61s ± 3%     2.55s ± 5%  -2.55%  (p=0.003 n=20+20)
FmtFprintfEmpty-12          52.8ns ± 6%    53.6ns ± 6%    ~     (p=0.228 n=20+20)
FmtFprintfString-12          177ns ± 4%     177ns ± 4%    ~     (p=0.280 n=20+20)
FmtFprintfInt-12             162ns ± 5%     162ns ± 3%    ~     (p=0.347 n=20+20)
FmtFprintfIntInt-12          277ns ± 7%     273ns ± 4%  -1.62%  (p=0.005 n=20+20)
FmtFprintfPrefixedInt-12     237ns ± 4%     242ns ± 4%  +2.13%  (p=0.005 n=20+20)
FmtFprintfFloat-12           315ns ± 4%     312ns ± 4%  -0.97%  (p=0.001 n=20+20)
FmtManyArgs-12              1.11µs ± 3%    1.15µs ± 4%  +3.41%  (p=0.004 n=20+20)
GobDecode-12                8.50ms ± 7%    8.53ms ± 7%    ~     (p=0.429 n=20+20)
GobEncode-12                6.86ms ± 9%    6.93ms ± 7%  +0.93%  (p=0.030 n=20+20)
Gzip-12                      326ms ± 4%     329ms ± 4%  +0.98%  (p=0.020 n=20+20)
Gunzip-12                   43.3ms ± 3%    43.8ms ± 9%  +1.25%  (p=0.003 n=20+20)
HTTPClientServer-12         72.0µs ± 3%    71.5µs ± 3%    ~     (p=0.053 n=20+20)
JSONEncode-12               17.0ms ± 6%    17.3ms ± 7%  +1.32%  (p=0.006 n=20+20)
JSONDecode-12               64.2ms ± 4%    63.5ms ± 3%  -1.05%  (p=0.005 n=20+20)
Mandelbrot200-12            4.00ms ± 3%    3.99ms ± 3%    ~     (p=0.121 n=20+20)
GoParse-12                  3.74ms ± 5%    3.75ms ± 9%    ~     (p=0.383 n=20+20)
RegexpMatchEasy0_32-12       104ns ± 4%     104ns ± 6%    ~     (p=0.392 n=20+20)
RegexpMatchEasy0_1K-12       358ns ± 3%     361ns ± 4%  +0.95%  (p=0.003 n=20+20)
RegexpMatchEasy1_32-12      86.3ns ± 5%    86.1ns ± 6%    ~     (p=0.614 n=20+20)
RegexpMatchEasy1_1K-12       523ns ± 4%     518ns ± 3%  -1.14%  (p=0.008 n=20+20)
RegexpMatchMedium_32-12      137ns ± 3%     134ns ± 4%  -1.90%  (p=0.005 n=20+20)
RegexpMatchMedium_1K-12     41.0µs ± 3%    40.6µs ± 4%  -1.11%  (p=0.004 n=20+20)
RegexpMatchHard_32-12       2.13µs ± 4%    2.11µs ± 5%  -1.31%  (p=0.014 n=20+20)
RegexpMatchHard_1K-12       64.1µs ± 3%    63.2µs ± 5%  -1.38%  (p=0.005 n=20+20)
Revcomp-12                   555ms ±10%     548ms ± 7%  -1.17%  (p=0.011 n=20+20)
Template-12                 84.2ms ± 5%    88.2ms ± 4%  +4.73%  (p=0.000 n=20+20)
TimeParse-12                 365ns ± 4%     371ns ± 5%  +1.77%  (p=0.002 n=20+20)
TimeFormat-12                361ns ± 4%     365ns ± 3%  +1.08%  (p=0.002 n=20+20)
[Geo mean]                  64.7µs         64.8µs       +0.19%

Change-Id: Ib043a7a0d18b588b298873d60913d44cd19f3b44
Reviewed-on: https://go-review.googlesource.com/19887
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>