Remove two "unsupported" tests from gdb.dwarf2/imported-unit.exp

gdb.dwarf2/imported-unit.exp yields two "unsupported" results but then
carries on.

These tests look for psymtabs, which haven't been used by the DWARF
reader since the introduction of the cooked index.

This patch removes these tests and also the supporting function
psymtabs_p, which is no longer used.