[gdb/testsuite] Fix c++/14186 kpass in cpexprs.exp

With gdb.cp/cpexprs.exp, we see:
...
KPASS: gdb.cp/cpexprs.exp: p CV::m(int) const (PRMS c++/14186)
KPASS: gdb.cp/cpexprs.exp: p CV::m(int) volatile (PRMS c++/14186)
KPASS: gdb.cp/cpexprs.exp: p CV::m(int) const volatile (PRMS c++/14186)
...

The tests have been KPASSing since Sept 4 2017, due to commit 3693fdb3c8
'Make "p S::method() const::static_var" work too'.

Fix this by removing the corresponding kfail.

gdb/testsuite/ChangeLog:

2020-02-19  Tom de Vries  <tdevries@suse.de>

	* gdb.cp/cpexprs.exp: Remove c++/14186 kfail.