Remove no-longer-relevant test case.

Remove no-longer-relevant test case.

collate.icu.utf8.sql was exercising the recording of a collation
dependency for an enum comparison expression, but such an expression
should never have had any collation dependency in the first place.
After I fixed that in commit c402b02b9, the test started failing.
We don't need to test that scenario anymore, so just remove the
now-useless test steps.

(This test case is new in HEAD, so no need to back-patch.)

Discussion: https://postgr.es/m/3044030.1618261159@sss.pgh.pa.us
Discussion: https://postgr.es/m/HK0PR01MB22744393C474D503E16C8509F4709@HK0PR01MB2274.apcprd01.prod.exchangelabs.com