efi_loader: make efi_remove_protocol() static

A previous patch is removing the last consumer of efi_remove_protocol().
Switch that to static and treat it as an internal API in order to force
users install and remove protocols with the appropriate EFI functions.

It's worth noting that we still have files using efi_add_protocol().  We
should convert all these to efi_install_multiple_protocol_interfaces()
and treat efi_add_protocol() in a similar manner

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>