Calculating a digest response header for MD5 without qop is simply a matter of concatenating the HA1, nonce, and HA2:
response = MD5(HA1, nonce, HA2)
A1 is calculated as
HA1 = MD5(username ":" realm ":" password)
A2 is:
HA2 = MD5(method ":" uri)
Note that this means you can store just the HA1, you don't need to keep the plaintext password in order to be able to authenticate users. Of course, the HA1 can still be used for authenticating against that realm and user, but it doesn't reveal the plain text password (which may be in use elsewhere, too).