Remove deprecated Event Watcher configration in .pipe dir

[ffjlabo]

What would you like to be added:

Why is this needed:

[t-kikuc]

Let's deprecate this feature. The process may be (the version is not determined):

1. Announce the future deprecation when releasing v0.49.0
2. Remove code&docs in v0.50.0

I thinks the change will be like this:

• code: https://github.com/pipe-cd/pipecd/commit/b7685544e958120a2f1e4f559ed3a072b8aab029
• docs: https://github.com/pipe-cd/pipecd/commit/bc48112cc65fab3bc4fbee18f8ecc38a28f71908

[t-kikuc]

First of all, let's show warning log 👍 See https://github.com/pipe-cd/pipecd/issues/5168