Remove Firebase/Firestore dependency in .podspec file

[sanghyeok-kim]

Context

Currently, Firebase/FireStore includes this library as a dependency in its podspec file,

s.dependency "Firebase/Firestore"

but it is not used anywhere.
And it's not just unused, it causes compile errors with objc code in existing projects.

For example, the count property in the FIRAggregateQuerySnapshot.mm code of Firebase/FireStore causes a compile error due to a conflict with the existing Objc's count.

- (NSNumber *)count {
  return (NSNumber *)[self valueForAggregateField:[FIRAggregateField aggregateFieldForCount]];
}

Error message

Multiple methods named 'count' found with mismatched result, parameter type, or attributes

Changes

Therefore, I propose removing this unnecessary dependency to avoid conflicts with existing Objc code and prevent compile errors.