[core] Fixed build with the old RCV buffer

[maxsharabayko]

The new function CUDT::dropTooLateUpTo(..) added in PR #2214 is expected to work only with the new receiver buffer, thus breaking the build if the old receiver buffer is enabled.
This PR enables the function only if the new receiver buffer is used.

[codecov-commenter]

Codecov Report

Merging #2215 (160af39) into master (1111cbd) will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2215      +/-   ##
==========================================
+ Coverage   65.60%   65.66%   +0.05%     
==========================================
  Files          95       95              
  Lines       19489    19489              
==========================================
+ Hits        12786    12797      +11     
+ Misses       6703     6692      -11     

Impacted Files	Coverage Δ
srtcore/core.cpp	59.31% <0.00%> (-0.29%)	⬇️
srtcore/core.h	79.01% <ø> (ø)
srtcore/window.cpp	81.81% <0.00%> (-1.30%)	⬇️
srtcore/buffer_rcv.cpp	66.50% <0.00%> (-0.25%)	⬇️
srtcore/api.cpp	51.37% <0.00%> (-0.22%)	⬇️
srtcore/buffer.cpp	65.36% <0.00%> (+0.60%)	⬆️
srtcore/congctl.cpp	80.72% <0.00%> (+1.56%)	⬆️
srtcore/queue.cpp	81.27% <0.00%> (+1.59%)	⬆️
srtcore/list.cpp	75.34% <0.00%> (+3.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1111cbd...160af39. Read the comment docs.