[gdb/testsuite] Clean standard_output_file dir in gdb_init

[gdb/testsuite] Clean standard_output_file dir in gdb_init

In commit e2adba909e7 ("[gdb/testsuite] Clean up before compilation in
gdb.ada/call-no-debug.exp") I added some code in the test-case to remove some
files at the start of the test-case:
...
remote_file host delete [standard_output_file prog.o]
remote_file host delete [standard_output_file prog.ali]
...

Replace this with cleaning up the entire directory instead, for all
test-cases.

Tested on x86_64-linux.

Suggested-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Tom Tromey <tom@tromey.com>