cp-completion-aliases.exp: Use test_gdb_complete_{unique,multiple}

cp-completion-aliases.exp: Use test_gdb_complete_{unique,multiple}

gdb.linespec/cp-completion-aliases.exp is calling
test_gdb_complete_{tab,cmd}_unique and
test_gdb_complete_{tab,cmd}_multiple separately for each use case.
I.e., testing once for TAB completion and once for the "complete"
command.  There's no need to do that explicitly and separately, we
have wrapper procedures to do that for us.

gdb/testsuite/ChangeLog:
2020-05-27  Pedro Alves  <palves@redhat.com>

	* gdb.linespec/cp-completion-aliases.exp: Remove readline_is_used
	check.  Use test_gdb_complete_unique instead of
	test_gdb_complete_tab_unique + test_gdb_complete_cmd_unique.  Use
	test_gdb_complete_multiple instead of
	test_gdb_complete_tab_multiple + test_gdb_complete_cmd_multiple.