Remove read_file_to_str helper

[mxpv]

Use fs::read_to_string from stdlib/tokio instead.

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (cc445f5) 38.08% compared to head (8753903) 38.02%.

Files	Patch %	Lines
crates/shim/src/asynchronous/util.rs	20.00%	4 Missing :warning:
crates/runc-shim/src/runc.rs	0.00%	2 Missing :warning:
crates/shim/src/asynchronous/mod.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
- Coverage   38.08%   38.02%   -0.06%     
==========================================
  Files          55       55              
  Lines        5099     5086      -13     
==========================================
- Hits         1942     1934       -8     
+ Misses       3157     3152       -5     

Flag	Coverage Δ
unittests	38.02% <12.50%> (-0.06%)	:arrow_down:

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