refactor: remove duplicate terminus checking code

[tserg]

What I did

Remove check_single_exit and is_return_from_function which duplicates is_terminus_node/check_for_terminus.

also fixes #3265

How I did it

Delete

How to verify it

Tests still pass

Commit message

refactor: remove duplicate terminus checking code

remove `check_single_exit` and `is_return_from_function` which duplicate
functionality in `is_terminus_node`/`check_for_terminus`.

additionally rewrite termination checking routine to be simpler, and
also fix an outstanding analysis bug where the following program would
not be rejected:

```vyper
@external
def foo(a: bool) -> uint256:
    if a:
        return 1
    else:
        return 2
    pass  # unreachable
```

Description for the changelog

Remove duplicate check for terminus node.

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[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (9cf66c9) 84.18% compared to head (d5c94a9) 84.20%.

Files	Patch %	Lines
vyper/ast/nodes.py	90.90%	1 Missing and 1 partial ⚠️
vyper/semantics/analysis/local.py	94.44%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3541      +/-   ##
==========================================
+ Coverage   84.18%   84.20%   +0.01%     
==========================================
  Files          92       92              
  Lines       13137    13133       -4     
  Branches     2928     2926       -2     
==========================================
- Hits        11059    11058       -1     
+ Misses       1659     1655       -4     
- Partials      419      420       +1     

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[charles-cooper]

@tserg could you take a look at the merge conflict?

[charles-cooper]

looks like the following program is (correctly) rejected on master but allowed on this branch:

@external
def foo():
    return
    return

[charles-cooper]

looks like the following program is (correctly) rejected on master but allowed on this branch:

@external
def foo():
    return
    return

i fixed this and also rewrote the check_terminator routine to be much simpler. i also fixed #3265 at the same time (see: 7b2c74b#diff-26eeb855b192798bd168b623451c4787dcee61bb51b19999cfc6a97881d6feb5R90-R91)