Remove bgzf_flush

[fgvieira]

Remove unnecessary bgzf_flush, since it is already called in bgzf_close and triggers a warning:

main_dfit.cpp: In function 'int main_dfit(int, char**)':
main_dfit.cpp:895:15: warning: ignoring return value of 'int bgzf_flush(BGZF*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  895 |     bgzf_flush(fpfpfp);//<- is flush needed?
      |     ~~~~~~~~~~^~~~~~~~
main_dfit.cpp:905:17: warning: ignoring return value of 'int bgzf_flush(BGZF*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  905 |       bgzf_flush(bootfp);//<- is flush needed?
      |       ~~~~~~~~~~^~~~~~~~

[ANGSD]

I only think flush is needed when using the peek