Remove Default Prylabs ETH1 Endpoint

[nisdas]

What type of PR is this?

Feature Addition

What does this PR do? Why is it needed?

• Removes the default eth1 endpoint which is hosted by Prysmatic Labs.
• Change error message to state consequences of not running your own eth1 node.

Which issues(s) does this PR fix?

Fixes #6884

Other notes for review

[codecov]

Codecov Report

Merging #7120 into master will increase coverage by 1.64%.
The diff coverage is 70.96%.

@@            Coverage Diff             @@
##           master    #7120      +/-   ##
==========================================
+ Coverage   60.07%   61.71%   +1.64%     
==========================================
  Files         323      408      +85     
  Lines       27422    32152    +4730     
==========================================
+ Hits        16473    19842    +3369     
- Misses       8733     9485     +752     
- Partials     2216     2825     +609     

[terencechain]

This looks good.

Can you also update the doc or add an issue for updating the doc to reflect this change? (If we need to) https://github.com/prysmaticlabs/documentation