By default, Rigify's modules are tailored for soft body animations. It uses intermediate bones and bendy bones to simulate things such as how a human hand rotation gradually affects the flesh before the joint.
Non-destructivbe change in the metarig
One way to avoid this on mechanical rigs, is to go in Pose mode on the metarig, select the Rigify sub-rig's parent (the parent of the limb module, such as the first arm bone, leg, finger, ... You know you got the right one if its Bone data tab shows a Rig Type in the Rigify Type panel), and set the limb segment and b-bone segment to 1. Then you can re-generate the full feature rig.
Example for an arm:
(You can see the result in the "DEF" layer.)
Manual destructive change in the final rig
If you find modules that have limb/b-bone segments, but you can't change them in the metarig ; or if you need to keep some of them and remove some others within a single module, you need to do the change on the final rig. But it will be overridden any time you re-generate the rig from the metarig.
B-bones are the easiest to handle: in the "DEF" bone group, select any bone you want, and set their b-bone segment number in the Bone data tab > Bendy Bones panel:
To remove segments, it's a bit harder because they are built on different groups and have constraints to update.
- In the DEF bone group, select the middle point between the two bones, and hit ⎈ CtrlX to dissolve it, turning the two bones into one.
- In pose mode, select the bone you just un-segmented, update its Stretch-to constraint to target the correct bone (usually the next bone in hierarchy, tweak bone if any), then hit the Original Lenght's reset button:
- In the "MCH" and any eventual "Tweak" bone group (my example is the right arm, so "Arm.R (Tweak)"), delete the unnecessary tweak controller bone and its MCH bone.
