apexer: remove --do_not_check_keyname

Since fe4ff29b4d47fa8ae242fb7e084215d3afada4b5 (in system/apex), apexer
doesn't check keyname (--key vs --manifest.name). So, passing
--do_not_check_keyname doesn't make any sense.

This is a preparation to remove (or deprecate) "apex_name" property from
"apex" module.

Bug: 267581665
Test: m
Change-Id: I3737d1e36d305375a523a23a6bbeb2d23586e578