powerpc: Remove obsoleted declarations for use_cop and drop_cop

The use_cop() and drop_cop() have been removed since
commit 6ff4d3e9 ("powerpc: Remove old unused icswx based
coprocessor support"), now they are useless, so remove them.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240822130609.786431-5-cuigaosheng1@huawei.com