Remove ComposeFoundationFlags.isPausableCompositionInPrefetchEnabled

The pausable composition prefetching feature has graduated to stable and the temporary feature flag is no longer needed. This change removes the flag, its associated API tracking, and the legacy full-composition codepath in LazyLayoutPrefetchState.

Test: ./gradlew :compose:foundation:foundation:testAndroidHostTest
Relnote: "Removed temporary feature flag ComposeFoundationFlags.isPausableCompositionInPrefetchEnabled."
Fixes: 455589928
TAG=agy
CONV=44c0995f-c6ef-499d-ad1d-3e831fb6fa86
Change-Id: Ifff7fc0a19fda40684f439b31db09cb0d2e05f34