c++: Fix templated convertion operator demangling

Instantiations of templated conversion operators failed to demangle
for cases such as 'operator X<int>', but worked for 'operator X<int>
&', due to thinking the template instantiation of X was the
instantiation of the conversion operator itself.

	libiberty/
	* cp-demangle.c (d_print_conversion): Remove incorrect
	template instantiation handling.
	* testsuite/demangle-expected: Add testcases.