loggers: revert useless exception (#15789)

Author: Borda

src/pytorch_lightning/loggers/tensorboard.py

@@ -38,7 +38,6 @@
 log = logging.getLogger(__name__)
 
 _TENSORBOARD_AVAILABLE = RequirementCache("tensorboard")
-_TENSORBOARDX_AVAILABLE = RequirementCache("tensorboardX")
 if TYPE_CHECKING:
     # assumes at least one will be installed when type checking
     if _TENSORBOARD_AVAILABLE:
@@ -114,10 +113,6 @@ def __init__(
         sub_dir: Optional[_PATH] = None,
         **kwargs: Any,
     ):
-        if not _TENSORBOARD_AVAILABLE and not _TENSORBOARDX_AVAILABLE:
-            raise ModuleNotFoundError(
-                "Neither `tensorboard` nor `tensorboardX` is available. Try `pip install`ing either."
-            )
         super().__init__()
         save_dir = os.fspath(save_dir)
         self._save_dir = save_dir