README.md: Remove mention of libtommath
README.md: Remove mention of libtommath

Seems to me libtommath is currently vendored in third_party and there
seems to be no build system configuration option to use the system
version over the vendored one. As such, there is no need to install it
using a distribution package manager.

See also: https://github.com/justinethier/cyclone/issues/458

justinethier/cyclone-bootstrap,justinethier/cyclone-bootstrap