Fix test-cp-name-parser build, unused variable

  $ make test-cp-name-parser
  ...
  CXX    test-cp-name-parser.o
  src/gdb/cp-name-parser.y: In function ‘int gdb::main(int, char**)’:
  src/gdb/cp-name-parser.y:2137:6: error: unused variable ‘len’ [-Werror=unused-variable]
    int len;
	^~~
  cc1plus: all warnings being treated as errors

gdb/ChangeLog:
2019-03-12  Pedro Alves  <palves@redhat.com>

	* cp-name-parser.y (main): Remove unused 'len' variable.