Simplify regex used in is_valid_smt_identifier

There are 2 simplifications here.
1) A swap to a normal string literal, rather that a raw string literal,
as there is only a single character which needs escaping. 2) A change to
remove the `^` and `$` for matching the start and end of the string.
These were redundant, because `std::regex_match` already requires the
regex to match the full string rather than a partial sub-match.

This simplification is already tested by the "smt_identifier_termt
construction" test in the smt_terms unit tests.

Author: thomasspriggs

src/solvers/smt2_incremental/smt_terms.cpp

@@ -52,7 +52,7 @@ static bool is_valid_smt_identifier(irep_idt identifier)
 {
   // The below regex matches a complete string which does not contain the `|`
   // character. So it would match the string `foo bar`, but not `|foo bar|`.
-  static const std::regex valid{R"(^[^\|]*$)"};
+  static const std::regex valid{"[^\\|]*"};
   return std::regex_match(id2string(identifier), valid);
 }