refactor: remove local variables, use static constants (googleapis#2322)

* refactor: remove local variables, use constants

These are already defined in the base class AbstractStatementParser

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/SpannerStatementParser.java

@@ -213,9 +213,6 @@ String removeStatementHint(String sql) {
   @InternalApi
   @Override
   ParametersInfo convertPositionalParametersToNamedParametersInternal(char paramChar, String sql) {
-    final char SINGLE_QUOTE = '\'';
-    final char DOUBLE_QUOTE = '"';
-    final char BACKTICK_QUOTE = '`';
     boolean isInQuoted = false;
     char startQuote = 0;
     boolean lastCharWasEscapeChar = false;