Remove "ada-exceptions" from -list-features output.

Remove "ada-exceptions" from -list-features output.

Now that the -info-gdb-mi-command is available, there is no need for
this entry. The entry and associated new commands were added recently
enough that no front-end out there should be depending on it yet.

gdb/ChangeLog:

        * mi/mi-main.c (mi_cmd_list_features): Remove "ada-exceptions".

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Miscellaneous Commands): Remove the
        documentation of the "ada-exceptions" entry.