How do I remove Received-headers from emails?

FWIW, this is a bad idea for Received headers but here's the answer anyways. womble mentioned content_filter as a recommended solution. IMHO that's overkill, Postfix supports this natively via header_checks.

/etc/postfix/main.cf:

header_checks = regexp:/etc/postfix/header_checks

/etc/postfix/header_checks:

/^Received:.*\[127\.0\.0\.1\]/         IGNORE
/^Received:.*\from apache@localhost/   IGNORE

Every header is useless -- until the day you really actually need that information to diagnose some irritating problem.

If you're dead-set on doing this, then you want to look at the content_filter parameter.