Drop redundant conditions around maybeAddImport and maybeRemoveImport

maybeAddImport only adds an import when the type is referenced, and maybeRemoveImport only removes one when the type is no longer referenced, so wrapping these calls in an if statement is redundant. Unwraps if statements without an else that contain only such calls, when the condition and arguments have no side effects and the condition does not guard the arguments. Guards comparing trees by identity, such as if (cu != c), are kept, as are calls passing onlyIfReferenced as anything other than true.