Make least request available by default (remove GRPC_EXPERIMENTAL_ENABLE_LEAST_REQUEST)

[atollena]

The gRFC https://github.com/grpc/proposal/blob/master/A48-xds-least-request-lb-policy.md states that the environment variable will be removed when the feature has proven stable. The last substantial changes to least request have been made in #6603 (1.58, Sep 2023).

We have successfully used least request in production for about 1 year, only discovering one minor issue last summer (fixed in #7467, 1.66).

We should consider removing the environment variable and always making least request available.

[arjan-bal]

Based on the comment from Eric assigning to @dfawley to share the result of the discussion.

[davinci26]

+1 on what @atollena is saying, we are also using it in prod for 6m+ with no issues

[dfawley]

#8253 is merged to enable it by default. Let's leave this open to track removing the flag after the 1.73 release.