[TASK] added documentation for option in ext. manager

Added the documentation of the option "UseHttpToFetch".
Remove "EncodeHeader" option and clean up code

Resolves: #60048
Releases: 4.0
Change-Id: I6a3fa0842ce0fb68dfe6b2c6dd3113bf813fe773
Reviewed-on: https://review.typo3.org/31280
Reviewed-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>
Tested-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>