[Ur] Redundant duplicate entry in urweb.lex

[Gabriel Riba]

In urweb.lex two entries check for the same token:

<INITIAL> "CURRENT_TIMESTAMP" => (Tokens.CURRENT_TIMESTAMP (pos yypos, 
pos yypos + size yytext));

<INITIAL> "CURRENT_TIMESTAMP" => (Tokens.CURRENT_TIMESTAMP (pos yypos, 
pos yypos + size yytext));