Fix(subprocess): fully remove tree-sitter dependency

[adridevlyon]

Description

In #577 there was an attempt to remove tree-sitter dependency that's not used by neotest itself

Problem is : there is still some code requiring it and it causes a crash (at least using Neovim 0.12.1 with Lazy) because

• nvim-treesitter is not found neither in the sub-folders nor in the global lua installation
• no root path is provided to resolve this module since #577

Changes

• remove the code that's related to nvim-treesitter and crashes
• adapt the config in the readme

Remaining question

@rcarriga I can see that there is still a mention of nvim-treesitter in the "Parsing Tests" part of the readme, especially for a code example. To be fair, I have no idea how this example should be adapted, would you mind help me on this before merging this PR?

Fixes : #597