Remove seq_split regex factorization module

Remove the seq_split (regex sigma-splitting / factorization) module and all
code that uses it:
- delete src/ast/rewriter/seq_split.{h,cpp} and src/test/seq_split.cpp
- drop the m_split member, include, and split/simplify_split/split_membership
  wrappers from seq_rewriter
- remove the regex factorization propagation block from seq_regex.cpp
- remove the seq.regex_factorization_enabled/threshold parameters and their
  theory_seq_params fields
- drop the files from the rewriter/test CMake lists and the test registry

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 57b9b87e-950a-49ea-bbb3-ed585646a5a9

src/test/main.cpp

@@ -198,7 +198,6 @@
     X(ho_matcher) \
     X(finite_set) \
     X(finite_set_rewriter) \
-    X(seq_split) \
     X(seq_regex_bisim) \
     X(term_enumeration) \
     X(lcube) \