Remove the high-level GIF tools (HDFGroup#4976)

The high-level GIF tools, h52gif and gif2h5, have unfixed CVE issues
(with no proof-of-concept files). They are not critical tools, are not
well maintained, and are an odd fit for building with the library.
Because of this, they have been removed. We may move them to a separate
repository in the future.

release_docs/RELEASE.txt

@@ -377,13 +377,6 @@ New Features
 
     Tools:
     ------
-    - Added h5dump command option to set the floating point format for long double
-
-      The new option is --lformat, which allows the user to set the
-      floating point format for long double. The default format is %Lg.
-      There is already an option --format to set the floating point format
-      for double and float. The default format is %g.
-
     - Remove the high-level GIF tools
 
       The high-level GIF tools, h52gif and gif2h5, have unfixed CVE issues