gdb/testsuite: remove get_compiler_info calls from gdb.exp and dwarf.exp

We don't need to call get_compiler_info before calling
test_compiler_info; test_compiler_info includes a call to
get_compiler_info.

This commit cleans up lib/gdb.exp and lib/dwarf.exp a little by
removing some unneeded calls to get_compiler_info.  We could do the
same cleanup throughout the testsuite, but I'm leaving that for
another day.

There should be no change in the test results after this commit.