libgomp C++ testsuite: Don't compute 'blddir' twice

It has already been set in 'libgomp/testsuite/lib/libgomp.exp:libgomp_init'.

	libgomp/
	* testsuite/libgomp.c++/c++.exp (blddir): Don't set.
	* testsuite/libgomp.oacc-c++/c++.exp (blddir): Likewise.