Makefile: remove an out-of-date comment

Remove a comment referring to a caveat that hasn't been applicable
since 18b0fc1ce1 ("Git.pm: Kill Git.xs for now", 2006-09-23).

At the time of 8d7f586f13 ("Git.pm: Support for perl/ being built by a
different compiler", 2006-06-25) some of the code in perl would be
built by a C compiler, but support for that went away a few months
later in 18b0fc1ce1 discussed above.

Since my 20d2a30f8f ("Makefile: replace perl/Makefile.PL with simple
make rules", 2017-12-10) the perl/ directory doesn't even have its own
build process.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -591,9 +591,6 @@ SPATCH_FLAGS = --all-includes --patch .
 
 ### --- END CONFIGURATION SECTION ---
 
-# Those must not be GNU-specific; they are shared with perl/ which may
-# be built by a different compiler. (Note that this is an artifact now
-# but it still might be nice to keep that distinction.)
 BASIC_CFLAGS = -I.
 BASIC_LDFLAGS =