Bump rrule to v2.7.2 and remove its import workarounds

[danielrozenberg]

Separating this change out of #37042 so renovatebot can do its thing

[danielrozenberg]

omg huge gain in bundle size for amp-date-picker-1.0! https://github.com/ampproject/amphtml/pull/39322/checks?check_run_id=15560299270

[ampprojectbot]

Warning: disparity between this PR Percy build and its main build

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