Remove redundancy in stringComparison

Fixes #434

Author: zrgt

documentation/IDTA-01002-3/modules/ROOT/partials/bnf/grammar.bnf

@@ -24,9 +24,7 @@
 
 <stringComparison> ::= 
     ( ( "$starts-with" | "$ends-with" | "$contains" | "$regex") <ws> "(" <ws> <stringOperand> <ws> "," <ws> <stringOperand> <ws> ")" <ws> ) |
-    ( <stringOperand> <ws> <allComparisons> <ws> <stringOperand> <ws> ) |
-    ( <stringOperand> <ws> <allComparisons> <ws> <FieldIdentifierString> <ws> ) |
-    ( <FieldIdentifierString> <ws> <allComparisons> <ws> <stringOperand> <ws> )
+    ( <stringOperand> <ws> <allComparisons> <ws> <stringOperand> <ws> )
 
 <numericalComparison> ::= 
     ( <numericalOperand> <ws> <allComparisons> <ws> <numericalOperand> <ws> ) |