Remove feature flag related to valid hosts list

What does this MR do?

As described in #326125 (comment 615271604), we never properly used VALID_HOSTS_CACHE_KEY (which stored a subset of the @host_list which are only caught up with the current location). The issue with that list was that we were resetting it on the release_host, so it didn't stay longer than for a single fetch.

The decision was to simplify the code, removing unnecessary management of the additional cache key.
It was rolled out on the feature flag via: !65356 (diffs)
Rollout issue: #335109 (closed)

Now we need to remove the feature flag and cleanup the related code.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

N/A

Related to #335107 (closed)