Remove free() function from DataBuf

[hassec]

Small PR aiming to address #1542.

Instead of renaming free() I chose to remove it since we already have reset() in DataBuf which does the same when called with its default argument.

[codecov]

Codecov Report

Merging #1687 (12d56ae) into main (2c57f21) will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #1687      +/-   ##
==========================================
- Coverage   66.90%   66.90%   -0.01%     
==========================================
  Files         151      151              
  Lines       20786    20781       -5     
==========================================
- Hits        13907    13903       -4     
+ Misses       6879     6878       -1     

Impacted Files	Coverage Δ
include/exiv2/types.hpp	90.90% <ø> (+3.95%)	⬆️
src/types.cpp	95.98% <ø> (-0.05%)	⬇️
src/pngimage.cpp	78.90% <75.00%> (ø)
src/image.cpp	72.66% <100.00%> (ø)

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[piponazo]

LGTM, Thanks!