Remove unreachable OOM handling

[azkrishpy]

Issue #, if available: N/A

Description of changes:

The CRT allocators abort on failure rather than returning NULL — aws_mem_acquire,
aws_mem_calloc and aws_mem_acquire_many all route through AWS_PANIC_OOM, which is an
unconditional exit(-1). The OOM branches guarded by these checks were therefore unreachable.

Removes those checks and the error paths behind them, and folds a few acquire-then-zero pairs
into aws_mem_calloc. No assert is added in their place: it would be dead in debug and compiled
out in release, since the abort happens before control returns.

No behavior change. No new tests — the existing suite covers these paths and passes unchanged (745).

Part of the todos stack (#576). Base is todos, not main.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (da2a213) to head (275b769).

Files with missing lines	Patch %	Lines
source/proxy_connection.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            todos     #577      +/-   ##
==========================================
+ Coverage   79.84%   79.94%   +0.10%     
==========================================
  Files          28       28              
  Lines       12105    12086      -19     
==========================================
- Hits         9665     9662       -3     
+ Misses       2440     2424      -16     

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