kbuild: modinst: remove the multithread option from zstd compression

Parallel execution is supported by GNU Make:

  $ make -j<N> modules_install

It is questionable to enable multithreading within each zstd process
by default.

If you still want to do it, you can use the environment variable:

  $ ZSTD_NBTHREADS=<N> make modules_install
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

scripts/Makefile.modinst

@@ -146,7 +146,7 @@ quiet_cmd_gzip = GZIP    $@
 quiet_cmd_xz = XZ      $@
       cmd_xz = $(XZ) --check=crc32 --lzma2=dict=1MiB -f $<
 quiet_cmd_zstd = ZSTD    $@
-      cmd_zstd = $(ZSTD) -T0 --rm -f -q $<
+      cmd_zstd = $(ZSTD) --rm -f -q $<
 
 $(dst)/%.ko.gz: $(dst)/%.ko FORCE
 	$(call cmd,gzip)