Remove dead code from dwarf2_const_value_data

Remove dead code from dwarf2_const_value_data

dwarf2_const_value_data checks the size of the data like so:

   if (bits < sizeof (*value) * 8)
...
  else if (bits == sizeof (*value) * 8)
...
   else
...

However, 'bits' can only be 8, 16, 32, or 64.  And, because 'value' is
a LONGEST, which is alwasy 64-bit, the final 'else' can never be
taken.

This patch removes the dead code.  And, because this was the only
reason for a non-void return value, the return type is changed as
well.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680