Fix some non-functional inconsistencies in floating-point compliance

In the floating point compliance section in the spec,
there were some subtle discrepancies between the
introductory paragraphs and the pseudocode below it.
The changes in this commit aim to fix these. More
specifically:
- "fp64 arithmetic" was defined but not used. This
  phrase has now been used in the bullet below and
  in the pseudocode.
- "operation_fp64" and "operation_imp" functions
  were defined but not used elsewhere in the spec.
  These have been amended to the same name as the
  functions called in the pseudocode.
- An unecessary "." in the `output_bnd` defintion
  was removed from the pseudocode.

Signed-off-by: Luke Hutton <luke.hutton@arm.com>
Change-Id: I229bc81a7b3b577cb6c609dbaa102a77f1a8d604