[chisel5] Prune memlib, remove --repl-seq-mem

Remove ReplSeqMem and related passes.  Prune memlib down to only
Annotations and utilities that are used by other areas of the SFC
codebase.

Co-authored-by: Adam Izraelevitz <adam.izraelevitz@sifive.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>