Remove `gitaly.client_path` and fix config.toml

What does this MR do?

This removes unused gitaly.client_path and fixes a source installation of gitaly to reflect a specified path in a rake "gitlab:gitaly:install[/home/git/gitaly2,...]"

We do:

• Remove gitaly.client_path as Rails do not depend on it
• Use gitaly_dir as specified by gitlab:gitaly:install
• Remove fetch_env as it is unused

Related:

• Update config.toml.example of Gitaly: gitaly!3503 (merged)
• Switch to make install in general: #331247

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)

• I have added/updated documentation, or it's not needed. (Is documentation required?)

• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)

• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)

• I have self-reviewed this MR per code review guidelines.

• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)

• I have followed the style guides.

• Label as security and @ mention @gitlab-com/gl-security/appsec

• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods

• Security reports checked/validated by a reviewer from the AppSec team