[BUGFIX] Remove relation to old em in impexp

If a import file has extension dependencies the user was
redirected to the old em with CMD requestInstallExtensions.
Since the new extensionmanager hasn't this functionality,
remove this magic and show the message only.

Resolves: #43709
Releases: 6.2, 6.1, 6.0
Change-Id: I161d4a24135f0d011177f2d4006e4713c71a8fcb
Reviewed-on: https://review.typo3.org/27100
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn