libexternal: remove mEDIDs string

Remove mEDIDs string from the External class as it not needed
after parsing the string

Change-Id: I0a624857894a0b5401b33408c09b32ceb4f8a0d4
CRs-fixed: 549578
(cherry picked from commit 3cd94a77a098102aee6dbe8d1dc85c2ae21e7ae0)

Conflicts:
	libexternal/external.h