Summary:
When building master using release --gcc11, compilation fails because the variable ybIsJoinPath is set but not used.
The reason is the assert containing this variable is compiled out and the variable is only used in the assert.
The fix is to change the assert to an error.
Jira: DB-16847

Test Plan:
TestPgRegressHints
TestPgRegressExtension
TestPgRegressJoin
TestPgRegressAggregates
TestPgRegressYbBitmapScans
TestPgRegressMisc
TestPgRegressDistinctPushdown
TestPgRegressPlanner
TestPgRegressTAQO
TestPgRegressIndex
TestPgRegressPgIndex
TestPgRegressBackfillIndex
TestPgRegressThirdPartyExtensionsPgHintPlan

Reviewers: gkukreja, mihnea, jason

Reviewed By: gkukreja, jason

Subscribers: jason, yql

Differential Revision: https://phorge.dev.yugabyte.com/D44717