Simplify ada-exp.o rule

Simplify ada-exp.o rule

The ada-exp.o rule no longer needs to pass -Wno-old-style-definition
to the compiler, as this option has no meaning in C++.  So, This patch
simplifies the explicit ada-exp.o rule in the Makefile.  The rule is
still needed because, according to the comment, ada-exp.c may appear
in the srcdir.

gdb/ChangeLog
2018-09-04  Tom Tromey  <tom@tromey.com>

	* Makefile.in (GDB_WARN_CFLAGS_NO_DEFS): Remove.
	(ada-exp.o): Update.