fix: remove supportsExplain

The `supportsExplain()` method did not actually do anything useful and
returned the wrong result. The reason that it was not useful is that:
1. Parsers that do support the EXPLAIN keyword handle these as client-side statements.
   This means that they never go into the isQuery() method.
2. Parsers that do not support the EXPLAIN keyword cannot do anything with it anyways.

Author: olavloite

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

@@ -617,21 +617,13 @@ public boolean isUpdateStatement(String sql) {
     return statementStartsWith(sql, dmlStatements);
   }
 
-  protected abstract boolean supportsExplain();
-
   private boolean statementStartsWith(String sql, Iterable<String> checkStatements) {
     Preconditions.checkNotNull(sql);
     Iterator<String> tokens = Splitter.onPattern("\\s+").split(sql).iterator();
     if (!tokens.hasNext()) {
       return false;
     }
     String token = tokens.next();
-    if (supportsExplain() && token.equalsIgnoreCase("EXPLAIN")) {
-      if (!tokens.hasNext()) {
-        return false;
-      }
-      token = tokens.next();
-    }
     for (String check : checkStatements) {
       if (token.equalsIgnoreCase(check)) {
         return true;

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

@@ -46,15 +46,6 @@ Dialect getDialect() {
     return Dialect.POSTGRESQL;
   }
 
-  /**
-   * Indicates whether the parser supports the {@code EXPLAIN} clause. The PostgreSQL parser does
-   * not support it.
-   */
-  @Override
-  protected boolean supportsExplain() {
-    return false;
-  }
-
   @Override
   boolean supportsNestedComments() {
     return true;

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

@@ -46,15 +46,6 @@ Dialect getDialect() {
     return Dialect.GOOGLE_STANDARD_SQL;
   }
 
-  /**
-   * Indicates whether the parser supports the {@code EXPLAIN} clause. The Spanner parser does
-   * support it.
-   */
-  @Override
-  protected boolean supportsExplain() {
-    return true;
-  }
-
   @Override
   boolean supportsNestedComments() {
     return false;