Remove an unnecessary test in filesystem

TestFileSystemShouldInstalCoreVariantIfTargetBuildAppIsSet is removed.
The test was originally introduced with aosp/2445946, but the change was
effectively reverted by aosp/2562192. The test however has been green as
a side effect of a bug (b/335506668) which will be fixed soon.

Bug: 335506668
Bug: 268582372
Test: N/A
Change-Id: I62f4746c605eba0b2f0037089a53d65922d65ef3