Specialize value_cast error message for Ada

In Ada, the term for a cast is "type conversion".  AdaCore has been
carrying a local patch to specialize the error message in value_cast,
but it seemed fine to me for this to be part of gdb.  This also
removes a dead "return" statement.

gdb/ChangeLog
2019-07-10  Tom Tromey  <tromey@adacore.com>

	* valops.c (value_cast): Specialize error message for Ada.