pgsql: Revert ill-fated patch to release exclusive lock early after

Log Message:
-----------
Revert ill-fated patch to release exclusive lock early after vacuum
truncates a table. Introduces race condition, as shown by buildfarm
failures.

Modified Files:
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pgsql/src/backend/commands:
vacuumlazy.c (r1.97 -> r1.98)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.97&r2=1.98)