Removes a spurious file which is created by leveldb

The file "-.gcno" is created by leveldb in case configure was ran with "--enable-lcov" as follows:
  (1) the leveldb Makefile calls build_detect_platform
  (2) build_detect_platform compiles an input from <stdin> to figure out if  <cstdatomic> is present
  (3) gcc --coverage is used, which in turn creates the file "-.gcno"
The file disrupts the creation of coverage info and should be removed.