gdb/fortran: Remove some dead code from the parser

gdb/fortran: Remove some dead code from the parser

The Fortran parser contains some code that looks like it was probably
inherited from the C/C++ parser as it checks to see if the current
language is C++, which should never be true when we're in the Fortran
parser.

gdb/ChangeLog:

	* f-exp.y (yylex): Remove is_a_field_of_this local variable, and
	all uses as this was never set to anything but a zero value.