[ios] fix repo-internal symlink

[ganthern]

this seems to have been forgotten when the calendar project was created.

The fact that nobody noticed may suggest that this file is actually not needed anymore.

[vaf-hub]

The git history suggests that it is not needed anymore because we do not use the C implementation in iOS anymore but the rust implementation:

git log --oneline -S "rand_entropy.h" --name-only
8aa250304 [iOS] Move common code between apps
app-ios/TutanotaSharedFramework/TutanotaSharedFramework.h
8403acb06 [iOS] Creates Calendar project
app-ios/TutanotaSharedFramework/TutanotaSharedFramework.h
8018d82e8 [ios] Use SDK for Kyber impl
app-ios/TutanotaNotificationExtension/tutanota-notification-bridging-header.h
app-ios/tutanota/Sources/tutanota-Bridging-Header.h
492cfa157 [ios] Generate the xcode projects using XcodeGen
app-ios/tutanota.xcodeproj/project.pbxproj
ad83f751b Extended notification preview, change credential encryption
app-ios/TutanotaNotificationExtension/tutanota-notification-bridging-header.h
c7f7bd75c Fix iOS build issues
app-ios/tutanota.xcodeproj/project.pbxproj
app-ios/tutanota/Sources/tutanota-Bridging-Header.h

So the last reference to rand_entropy.h was removed here 8aa2503#diff-b1f8e8701c847a9558980eb28ef4d524fc464e4430f81658296282c5c2e6b9f1L9

I would just delete the symlink if that is correct.

[charlag]

If we delete the symlink and everything still works then we should just do it