global: remove caching mechanism

[mdonadoni]

REANA's current caching mechanism has been disabled by default for quite some time and it has some drawbacks:

• big disk usage, as the whole workspace is copied every time a new job needs to be cached
• slowness in job creation (see perf(cache): avoid caching jobs when the cache is disabled (#435) reana-job-controller#435)
• caching does not always work as expected and some tricky cases are not handled correctly
• this caching mechanism is only supported by serial workflows, and would not work with other engines as it does not handle multiple jobs running concurrently

For these reasons, we should consider removing this caching mechanism from REANA:

• remove all related code
• drop related columns in the database (migration script is needed)
• update docs (reana/reana-client)
• update API, e.g. do not accept CACHE option for serial workflows (breaking change!)
• delete the cache directory from current deployments of REANA