Simplify imenu setup for {cmake,dockerfile}-ts-modes

Simplify imenu setup for {cmake,dockerfile}-ts-modes

* lisp/progmodes/cmake-ts-mode.el (treesit-induce-sparse-tree,
treesit-node-child, treesit-node-start, cmake-ts-mode--imenu,
cmake-ts-mode--imenu-1): Remove.
(treesit-search-subtree): Declare.
(cmake-ts-mode--function-name): New function.
(cmake-ts-mode): Use it.

* lisp/progmodes/dockerfile-ts-mode.el (treesit-induce-sparse-tree,
treesit-node-start, dockerfile-ts-mode--imenu,
dockerfile-ts-mode--imenu-1): Remove.
(dockerfile-ts-mode--stage-name): New function.
(dockerfile-ts-mode): Use it.