remove erroneous PreserveObject call in String::wrap

[kevinushey]

Part of #1081.

This PR fixes an issue where String::wrap() would un-necessarily call Rcpp_PreserveObject(), thereby 'leaking' the SEXP returned to R (basically, not allowing the R GC to ever collect that object).

Unfortunately it doesn't fix #1081 but I do think it's at least part of the issue.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[codecov-io]

Codecov Report

Merging #1082 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
- Coverage   95.55%   95.54%   -0.01%     
==========================================
  Files          64       64              
  Lines        2787     2784       -3     
==========================================
- Hits         2663     2660       -3     
  Misses        124      124              

Impacted Files	Coverage Δ
inst/include/RcppCommon.h	100.00% <100.00%> (ø)