sql: fix wrong region allocation

This patch fixes an issue in generate_column_metadata(). Prior to this
patch, the number of variable-only expressions was counted incorrectly
when temporary memory was allocated on region to store their positions.
However, although this allocation was incorrect, this did not lead to
any problems due to the specifics of the region allocations.

This patch fixes this by removing the temporary memory allocation.

Closes #8763

NO_DOC=no user-visible changes
NO_TEST=no user-visible changes
NO_CHANGELOG=no user-visible changes

(cherry picked from commit d4f143ad)