[regalloc] Remove -consider-local-interval-cost

Discussed extensively on D98232. The functionality introduced in D35816
never worked correctly. In D98232, it was fixed, but, as it was
introducing a large compile-time regression, and the value of the
original patch was called into doubt, we disabled it by default
everywhere. A year later, it appears that caused no grief, so it seems
safe to remove the disabled code.

This should be accompanied by re-opening bug 26810.

Differential Revision: https://reviews.llvm.org/D121128