Removed CurlEpollFdTracker completely

[DrDet]

CurlEpollFdTracker was added as a fix of another problem but it didn't help (a37c782).
Moreover it led to another crash in runtime from free_runtime_environment, because iterating over the array like this:

kphp/runtime/curl.cpp

Line 1036 in 8a5a520

for (auto it : used_fds_) {

uses non-const iterator, which can mutate the array and invokes script memory allocations. In case of memory limit exceeded error, free_runtime_environment is always called, which in turn calls the code described above. And it leads to SIGSEGV in the end.