Remove dbSystem parameter from all exported functions

[XSAM]

Though db.system is a required attribute for Span, it is verbose to put it on the parameters. Users can directly use otelsql.WithAttributes option to add this attribute.

[codecov]

Codecov Report

Merging #80 (e5c683e) into main (a91b677) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main     #80     +/-   ##
=======================================
- Coverage   80.5%   80.3%   -0.2%     
=======================================
  Files         12      12             
  Lines        678     672      -6     
=======================================
- Hits         546     540      -6     
  Misses       107     107             
  Partials      25      25             

Impacted Files	Coverage Δ
config.go	90.0% <100.0%> (-1.7%)	⬇️
sql.go	79.2% <100.0%> (ø)

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