Eliminate the sizing template parameter N from CoalescingBitVector

Since the parameter is not used anywhere, and the default size of 16
apparently causes PR47359, remove it. This ensures that IntervalMap will
automatically determine the optimal size, using its NodeSizer struct.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D87044

(cherry picked from commit f26fc568)