prevent adding methods to the functions > and >=

[nsajko]

As documented, the intended way to implement > is to add a method to <. Similarly with >=. A package should never add a method to either > or >=.

The != function should also usually not get any new methods.

EDIT: some of the methods here behave differently than Julia's generic fallback. These methods are preserved with this change to prevent breakage.

[JeffreySarnoff]

Thank you.
This was done intentionally, to preserve whatever logic the Arb C library implementation imposes.
As ArbReal is an interval type, there are subtle issues with the comparisons. It may be that the default way of defining them matches the internals -- I do not know offhand. Your thoughts.

[nsajko]

I reverted all deletions that might impact behavior. Now this PR just deletes methods such that there is obviously no change in behavior.