SF: Remove misleading pacesetter choice fallback

Since Id5cb29c3cbaa8ed455a15d8be3a32e79a470cce5, the pacesetter display
is always specified to Scheduler::promotePacesetterDisplayLocked, so the
fallback to pick the first display as the pacesetter (and std::optional
plumbing) is dead code.

Bug: 329450361
Flag: EXEMPT refactor
Test: presubmit
Change-Id: I831de5eef94eaefe0682f9c87ffe76acd50171b0