Delete files MANIFESTO, BUGS and INSTALL (valkey-io#958)

The MANIFESTO is not Valkey's manifesto and it doesn't even mention open
source. Let's write another one later, or some other document about our
project principles.

The other two files are one-line files with no relevant info. They're
polluting the file listing at root level. It's the first thing you see
when you start exploring the repo for the first time.

Signed-off-by: Viktor Söderqvist <[email protected]>