Make explicit importing std.math unnecessary for pow expressions

Currently front-end does not enforce std.math module existence (done by #2358). Therefore, there's not also necessity to explicitly import std.math in user code.

By this change, a diagnostic regression will be naturally fixed:
[REG2.065] [Issue 12197](https://d.puremagic.com/issues/show_bug.cgi?id=12197) - 'invalid array operation' errror message with valid array operation