net: ipa: kill FILT_ROUT_CACHE_CFG IPA register

A recent commit defined a few IPA registers used for IPA v5.0+.
One of those was a mistake.  Although the filter and router caches
get *flushed* using a single register, they use distinct registers
(ENDP_FILTER_CACHE_CFG and ENDP_ROUTER_CACHE_CFG) for configuration.

And although there *exists* a FILT_ROUT_CACHE_CFG register, it is
not needed in upstream code.  So get rid of definitions related to
FILT_ROUT_CACHE_CFG, because they are not needed.

Fixes: 8ba59716 ("net: ipa: define IPA v5.0+ registers")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>