ice: remove ICE_F_PTP_EXTTS feature flag

The ICE_F_PTP_EXTTS feature flag is ostensibly intended to support checking
whether the device supports external timestamp pins. It is only checked in
E810-specific code flows, and is enabled for all E810-based devices. E822
and E823 flows unconditionally enable external timestamp support.

This makes the feature flag meaningless, as it is always enabled. Just
unconditionally enable support for external timestamp pins and remove this
unnecessary flag.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>