arm64: module: Remove unused plt_entry_is_initialized()

Since commit f1a54ae9 ("arm64: module/ftrace: intialize PLT at load
time"), plt_entry_is_initialized() is unused anymore , so remove it.

Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220929094134.99512-3-lihuafei1@huawei.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>