[Enhancement] Remove mmdet and mmcls from mminstall (open-mmlab#2642)

## Motivation

As the mmdet and mmcls are not very stabel, and mim can install repo
from source code, we remove them from mminstall and they won't be
installed automatically when run `mim install mmsegmentation`

## Modification

1. remove mmdet and mcls from mminstall
2. add explanation in faq

---------

Co-authored-by: MengzhangLI <[email protected]>

Author: MeowZheng

docs/en/notes/faq.md

@@ -17,7 +17,11 @@ The compatible MMSegmentation, MMCV and MMEngine versions are as below. Please i
 |        1.0.0rc1        | mmcv >= 2.0.0rc1, \<=2.0.0rc3> | MMEngine >= 0.1.0 |           mmcls>=1.0.0rc0           |          Not required          |
 |        1.0.0rc0        | mmcv >= 2.0.0rc1, \<=2.0.0rc3> | MMEngine >= 0.1.0 |           mmcls>=1.0.0rc0           |          Not required          |
 
-Notes: To install MMSegmentation 0.x and master branch, please refer to [the faq 0.x document](https://mmsegmentation.readthedocs.io/en/latest/faq.html#installation) to check compatible versions of MMCV.
+Notes:
+
+- MMClassification and MMDetatction are optional for MMSegmentation. If you didn't install them, `ConvNeXt` (required MMClassification) and MaskFormer, Mask2Former (required MMDetection) cannot be used. We recommend to install them with source code. Please refer to [MMClasssication](https://github.com/open-mmlab/mmclassification) and [MMDetection](https://github.com/open-mmlab/mmdetection) for more details about their installation.
+
+- To install MMSegmentation 0.x and master branch, please refer to [the faq 0.x document](https://mmsegmentation.readthedocs.io/en/latest/faq.html#installation) to check compatible versions of MMCV.
 
 ## How to know the number of GPUs needed to train the model
 

requirements/mminstall.txt

@@ -1,4 +1,2 @@
-mmcls>=1.0.0rc0
 mmcv>=2.0.0rc4
--e git+https://github.com/open-mmlab/[email protected]#egg=mmdet
 mmengine>=0.5.0,<1.0.0