[!!!][TASK] Extension manager: Drop "Download SQL Dump"

The em in "Installed Extensions" has a button "Download SQL Dump"
for all extensions that provide ext_tables.sql. On click, an sql
dump file is sent.

This feature is severely flawed:
* Dumps of extensions that add fields to existing tables contain
  a 'drop table' of these tables, the 'import into' statements are
  broken and only (try to) add these fields again. This easily
  leads to hazard in DB if importing such a dump.
* There are no charset specs and other meta data whatsoever in the dump.
* The dump is not dbal compatible, field definitions and imports
  are incomplete.

We assume nobody really used this feature in a sane way, even at
this prominent position in em. The lack of bug reports to this
broken feature and the fact there have been zero changes in this
area since main em refactoring years ago support this view.

There are way better options to retrieve proper data specifications:
* The list module has a csv export
* Ext:impexp supports export and import in a much better way
  including proper relation handling and other options.
* Low level db exports and backups should be done on cli or
  with more powerful guis like phpmyadmin or other db engine
  specific tools.

The feature is dropped without substitution.

The v8 backport of this patch will just remove the button from
the em list view, but keep all code.

Change-Id: Ia027e7498c5464af04c49675987a696ee3a06070
Resolves: #82148
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/53764


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>