remove nvim-treesitter branch option from gitmodules

[perrin4869]

At this point in time, https://github.com/nvim-treesitter/nvim-treesitter and https://github.com/nvim-treesitter/nvim-treesitter-textobjects still set their default branches to master, even though those are frozen.
I migrated to using their new main branches on #2982, by adding the branch option to the .gitmodules file here and here.
When the aforementioned repos finally migrate their default branches to main, this parameter becomes superfluous, so I need to remove it.

• Roadmap: Nvim-treesitter 1.0 nvim-treesitter/nvim-treesitter#4767
• Transition to main branch nvim-treesitter/nvim-treesitter-textobjects#772

[perrin4869]

nvim-treesitter was done in 6a19972, it was changed to default in nvim-treesitter/nvim-treesitter@d3218d9