cmListFileLexer: Remove extraneous UnquotedArgument rule
The rule
({UNQUOTED}|=|\[=*{UNQUOTED})({UNQUOTED}|[[=])*
is completely covered by
({MAKEVAR}|{UNQUOTED}|=|\[=*{LEGACY})({LEGACY}|[[=])*
so there's no reason to keep the first one. This makes the grammar simpler and makes the Flex-generated lexer slightly smaller.