[gdb/testsuite] Remove stale exec in gdb_compile_ada
    
    When running test-case gdb.ada/ptype_tagged_param.exp, I get:
    ...
    PASS: gdb.ada/ptype_tagged_param.exp: compilation foo.adb
    ...
    
    However, when I then de-install gnatmake and run again, I get the same result.
    This is due to a stale exec. After removing the stale exec, I get:
    ...
    UNSUPPORTED: gdb.ada/ptype_tagged_param.exp: compilation foo.adb
    ...
    
    Fix this removing the stale exec in gdb_compile_ada before compilation.
    
    Tested on x86_64-linux.
    
    gdb/testsuite/ChangeLog:
    
    2020-02-13  Tom de Vries  <tdevries@suse.de>
    
    	* lib/ada.exp (gdb_compile_ada): Delete stale exec before compilation.