QtJsonDb should not depend on libssl and libcrypto

If openssl header files (ssl-dev) are not installed, i.e. so Qt is compiled with -no-openssl, QtJsonDb fails to compile ungracefully:
../3rdparty/btree/src/btree_p.h:50:27: fatal error: openssl/sha.h: No such file or directory

And btree.pri adds LIBS += -lssl -lcrypto

But btree.c only needs this to find a function to compute a sha1 sum, and that is already present in qtbase, see QCryptographicHash. Hance, it is redundant and very clumsy to require ssl-dev and libs here.

(Another issue is that it presumably could rather use a non-cryptographic checksum function instead, e.g. murmurhash, which would save CPU cycles)