fix(instrumentation-pg): remove @opentelemetry/core from dependencies

[mothershipper]

Was introduced in commit a0003e76fc46afbbee2558a7d21906be7c9cb1d1 but all usages seem to have been removed.

By having this in the dependencies, we were getting peer-dep warnings on install as this version of core seems to be outdated.

Which problem is this PR solving?

• When we run installs including the pg instrumentation, we get errors related to a peer-dep required by an old version of core, ex:

$ yarn install
warning "@opentelemetry/instrumentation-pg > @opentelemetry/core@1.8.0" has incorrect 
   peer dependency "@opentelemetry/api@>=1.0.0 <1.4.0".

Short description of the changes

Removes the dependency as the dep is no longer used within the package. This should clean up the warning.

[codecov]

Codecov Report

Merging #1895 (aa277f5) into main (1e90a40) will increase coverage by 4.58%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1895      +/-   ##
==========================================
+ Coverage   91.47%   96.06%   +4.58%     
==========================================
  Files         145       14     -131     
  Lines        7423      914    -6509     
  Branches     1483      199    -1284     
==========================================
- Hits         6790      878    -5912     
+ Misses        633       36     -597     

see 131 files with indirect coverage changes