MAINT: Replace defusedxml with mffpy in _get_gains and remove _extract

[PragnyaKhandelwal]

Reference issue (if any)

Part of #13926 (Phase 1 — legacy helper cleanup).
Follows #13991 (which migrated _get_ep_info and _get_signalfname).

What does this implement/fix?

Replaces the defusedxml-backed _get_gains with mffpy-backed parsing and removes the internal _extract helper that was only ever called by _get_gains.
_get_gains previously parsed info[N].xml calibration data by walking raw XML nodes via defusedxml.minidom. It now delegates to mffpy.xml_files.XML.from_file() and reads the structured calibrations dict from get_content(), which is the same mffpy API already used throughout the module after #13991.
With _extract gone and _get_gains migrated, defusedxml is now fully eliminated from mne/io/egi/general.py and _soft_import for it is removed as well.

Additional information

_get_blocks and _block_r remain — they handle binary .bin block parsing which has no mffpy equivalent yet (targeted for Phase 2 / PNS migration).

[scott-huberty]

LGTM 🚀 !

Note that the code on main will only return 'GCAL' or 'ICAL' data, but our new PR could in theory return both. However I don't think this will be an issue, the code on main returns a dict, and so does the code in this PR.