feat(agent): update instrumentation to be compatible with MongoDB 2.x

[hahuja2]

This PR removes redundant Executable interface check which caused instrumentation to break for MongoDB 2.x because the Executable interface has been removed. This check is not needed because for all operations we implement, they will only be instrumented if execute method is called, so this makes current check for Executable interface redundant as this interface only defined execute method.

closes #1147

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	18/18 passing
SOAK	❌	141/142 passing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.35%. Comparing base (010ff93) to head (7f2f608).

Files with missing lines	Patch %	Lines
agent/lib_mongodb.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1154      +/-   ##
==========================================
+ Coverage   78.33%   78.35%   +0.02%     
==========================================
  Files         193      193              
  Lines       28238    28229       -9     
==========================================
  Hits        22120    22120              
+ Misses       6118     6109       -9     

Flag	Coverage Δ
agent-for-php-7.2	78.34% <ø> (ø)
agent-for-php-7.3	78.36% <ø> (ø)
agent-for-php-7.4	78.22% <ø> (ø)
agent-for-php-8.0	77.74% <0.00%> (+0.02%)	⬆️
agent-for-php-8.1	77.97% <0.00%> (+0.02%)	⬆️
agent-for-php-8.2	77.59% <0.00%> (+0.02%)	⬆️
agent-for-php-8.3	77.59% <0.00%> (+0.02%)	⬆️
agent-for-php-8.4	77.60% <0.00%> (?)
agent-for-php-8.5	76.93% <0.00%> (+0.02%)	⬆️

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