mgmt: fix make clean

"make clean" still thinks it needs to clean mgmt/httpserver, but it is
no longer there, so this breaks "make clean". Remove the command from
mgmt.git. A separate patch will readd it to osv.git.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>