Cleanup the MLIR LSP doc to remove references to the passes

[mehdi_amini]

Summary

The MLIR LSP server initially had plans to support running passes from the
client, but this was never implemented. The doc unnecessarily advise to
register passes and makes reference to LSP server having some interaction
with passes.

[rriddle]

Can you add more context to the description?

[mehdi_amini]

Add description

mlir/docs/Tools/MLIRLSP.md

 etc.
 
 ## MLIR LSP Language Server : `mlir-lsp-server`
 
 MLIR provides an implementation of an LSP language server for `.mlir` text files
 in the form of the `mlir-lsp-server` tool. This tool interacts with the MLIR C++
 API to support rich language queries, such as "Find Definition".
 
-### Supporting custom dialects and passes
+### Supporting custom dialects
 
 `mlir-lsp-server`, like many other MLIR based tools, relies on having the
 appropriate dialects registered to be able to parse in the custom assembly
 formats used in the textual .mlir files. The `mlir-lsp-server` found within the
-main MLIR repository provides support for all of the upstream MLIR dialects and
-passes. Downstream and out-of-tree users will need to provide a custom
+main MLIR repository provides support for all of the upstream MLIR dialects.
+Downstream and out-of-tree users will need to provide a custom
 `mlir-lsp-server` executable that registers the entities that they are
 interested in. The implementation of `mlir-lsp-server` is provided as a library,
-making it easy for downstream users to register their dialect/passes and simply
+making it easy for downstream users to register their dialect and simply
 call into the main implementation. A simple example is shown below:
 
 ```c++
 #include "mlir/Tools/mlir-lsp-server/MlirLspServerMain.h"
 
 int main(int argc, char **argv) {
   mlir::DialectRegistry registry;
   registerMyDialects(registry);
-  registerMyPasses();
   return mlir::failed(mlir::MlirLspServerMain(argc, argv, registry));
 }
 ```
 
 See the [Editor Plugins](#editor-plugins) section below for details on how to
 setup support in a few known LSP clients, such as vscode.
 
 ### Features