kbuild: remove PYTHON2 variable

Python 2 has retired. There is no user of this variable.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -423,7 +423,6 @@ INSTALLKERNEL  := installkernel
 DEPMOD		= /sbin/depmod
 PERL		= perl
 PYTHON		= python
-PYTHON2		= python2
 PYTHON3		= python3
 CHECK		= sparse
 BASH		= bash
@@ -474,7 +473,7 @@ CLANG_FLAGS :=
 
 export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE AS LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP OBJSIZE READELF PAHOLE LEX YACC AWK INSTALLKERNEL
-export PERL PYTHON PYTHON2 PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
+export PERL PYTHON PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS