[gdb/testsuite] Clean up before compilation in gdb.ada/call-no-debug.exp

[gdb/testsuite] Clean up before compilation in gdb.ada/call-no-debug.exp

Running test-case gdb.ada/call-no-debug.exp with target board unix/-m64 works
fine, but if we run it again with target board unix-m32, we run into:
...
gnatlink prog.ali -m32 -g -o prog^M
ld: i386:x86-64 architecture of input file `b~prog.o' is incompatible with \
  i386 output^M
...

This is due to compiling with no-force.

The test-case:
- first compiles pck.adb into pck.o (without debug info), and
- then compiles prog.adb and pck.o into prog (with debug info).

Using no-force in the second compilation make sure that pck.adb is not
compiled again, with debug info.

But it also means it will pick up intermediate files related to prog.adb from
a previous compilation.

Fix this by removing prog.o and prog.ali before compilation.

Tested on x86_64-linux.