MINOR: pattern: perform a single call to pat_delete_gen() under the expression

MINOR: pattern: perform a single call to pat_delete_gen() under the expression

When we're removing an element under the expression lock, we don't need
anymore to run over all ->delete() functions via the expressions, since
we know that the single function does it fine now. Note that at this
point, pattern->delete() is not used at all through out the code anymore.