docs: record the ENABLE_MFE_CONFIG_API removal in ADR 0001

ADR 0001 recorded the decision to gate the MFE config API behind
ENABLE_MFE_CONFIG_API.  That decision has been reversed, so replace the
bullet with the current state -- the API is always available -- while
keeping the history and linking the DEPR ticket that removed the
toggle.

See DEPR ticket: https://github.com/openedx/openedx-platform/issues/38959

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>