kbuild: remove PYTHON variable

Python retired in 2020, and some distributions do not provide the
'python' command any more.

As in commit 51839e29cb59 ("scripts: switch explicitly to Python 3"),
we need to use more specific 'python3' to invoke scripts even if they
are written in a way compatible with both Python 2 and 3.

This commit removes the variable 'PYTHON', and switches the existing
users to 'PYTHON3'.

BTW, PEP 394 (https://www.python.org/dev/peps/pep-0394/) is a helpful
material.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/Makefile b/Makefile
index b0e4767..89217e4 100644
--- a/Makefile
+++ b/Makefile
@@ -452,7 +452,6 @@
 INSTALLKERNEL  := installkernel
 DEPMOD		= depmod
 PERL		= perl
-PYTHON		= python
 PYTHON3		= python3
 CHECK		= sparse
 BASH		= bash
@@ -508,7 +507,7 @@
 
 export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
-export PERL PYTHON PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
+export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE