gdb/Makefile.in: remove ACLOCAL_AMFLAGS

aclocal picks up the relevant include paths from AC_CONFIG_MACRO_DIRS in
configure.ac, so there's no need to pass `-I ../config` here.

Passing `-I ../config` is actually annoying, because it makes the output
different between when the update is triggered by the maintainer mode
and when aclocal or autoreconf is ran with no special flags.  The
difference in the output is due to the order of include paths being
different.

Change-Id: I2c963876516570842f20b4a6a470867e7a941006
Approved-By: Tom Tromey <tom@tromey.com>