tools/build/make.py: Drop problematic Linux hack for bmake bootstrap

Since e2eeea75eb8b ("Merge bmake-20201117") missing/sys/cdefs.h has
been present in bmake, and _GNU_SOURCE seems to have been defined by
config.h for much longer than that, possibly for the entire time OS
cross-build support has been in-tree, so these are obsolete. Moreover,
since 79e02149fcb4 ("Fix dtrace tools bootstrap on non-FreeBSD after
OpenZFS import"), HAVE_STRLCAT and HAVE_STRLCPY have been defined by our
cross-build headers in order to placate DTrace tools (which is not the
right way to solve that problem, but motivates fixing this one). Commit
4fde40d9b540 ("Merge/update to bmake-20230126") changed the strlcpy.c in
bmake from including config.h directly to including make.h, which means
it includes string.h and thus sees these bogus definitions, causing it
to not define the strlcpy compat function on Linux even though it needs
to and thus failing to link. Thus, fix this whole mess by removing the
hack we no longer need.