[BUGFIX] Detection of image manipulation libs with open_basedir

If open_basedir is restricted to files, eg. /usr/bin/gm, detection
fails in install tool. The additional is_dir() check may fail then.
The patch removes it since @is_file() covers the same checks.

Resolves: #58466
Releases: 6.2
Change-Id: I5ab6a36ccd3d283a1dc1e672f7deddc2f0ea37c4
Reviewed-on: https://review.typo3.org/29822
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers