Remove one-time variable $pieces for terms_clauses

[ocean90]

$pieces is since [28736] only used in compact(). Having the extra variable makes the current code harder to read and can also confuse IDEs because they can't correctly evaluate whether a variable is used or not.

[boonebgorges]

In 38377:

Eliminate unnecessary variable in WP_Term_Query.

Props ocean90.
Fixes #37544.