[MemProf] Removed unused allocation type

Removes the 'notcoldandcold' allocation type summary
(de)serialization support added in D135714, after realizing that this
will never be generated in practice.

There are 2 uses of the allocation type keywords in the summary. One is
for the individual profiled memprof context summaries, and each context
can only be assigned a single type of hotness. The second is in the
clone version information produced by the MemProfContextDisambiguation
whole program step, and we only create a clone for a specific allocation
type.

Differential Revision: https://reviews.llvm.org/D149669