Replicate fix on PR #12643 for pledge report

[MegaphoneJon]

Overview

I saw the fix @jmcclelland applied in PR #12643. On a hunch, I grepped for that code and found it copy/pasted to another report. This PR cleans that up.

Before

"Pledged but not Paid" report only lets you filter by static groups.

After

"Pledged but not Paid" report can filter on smart groups; less code duplication.

Comments

This fix is identical to #12643.

[civibot]

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[yashodha]

@MegaphoneJon looks good. Can be merged when tests pass.

[yashodha]

@eileenmcnaughton merging this.

[eileenmcnaughton]

@yashodha thanks!