Remove density from mask nifti file template

As pointed out by @tsalo, this entity is not applicable to nifti files.
And as pointed out by @mattcieslak, dwi masks don't need bvecs and bvals.

Author: mguaypaq

src/derivatives/imaging.md

@@ -150,7 +150,8 @@ Template:
     sub-<label>/
         [ses-<label>/]
             anat|func|dwi/
-                <source_entities>[_space-<space>][_res-<label>][_den-<label>][_label-<label>][_desc-<label>]_mask.nii.gz
+                <source_entities>[_space-<space>][_res-<label>][_label-<label>][_desc-<label>]_mask.json
+                <source_entities>[_space-<space>][_res-<label>][_label-<label>][_desc-<label>]_mask.nii[.gz]
 ```
 
 A binary (1 - inside, 0 - outside) mask in the space defined by the [`space` entity](../appendices/entities.md#space).

src/schema/rules/files/deriv/imaging.yaml

@@ -52,7 +52,6 @@ anat_parametric_mask:
     $ref: rules.files.raw.anat.parametric.entities
     space: optional
     resolution: optional
-    density: optional
     label: optional
     description: optional
 
@@ -64,19 +63,21 @@ anat_nonparametric_mask:
     $ref: rules.files.raw.anat.nonparametric.entities
     space: optional
     resolution: optional
-    density: optional
     label: optional
     description: optional
 
 dwi_mask:
   $ref: rules.files.raw.dwi.dwi
   suffixes:
     - mask
+  extensions:
+    - .nii.gz
+    - .nii
+    - .json
   entities:
     $ref: rules.files.raw.dwi.dwi.entities
     space: optional
     resolution: optional
-    density: optional
     label: optional
     description: optional
 
@@ -88,7 +89,6 @@ func_mask:
     $ref: rules.files.raw.func.func.entities
     space: optional
     resolution: optional
-    density: optional
     label: optional
     description: optional