doc: remove mentioning of schemacode install as the third command

ATM, the preceding code snippet doesn't have a
third command, and the installation of
`bidsschematools` is done in the subsequent
paragraph.

Author: candleindark

CONTRIBUTING.md

@@ -239,9 +239,6 @@ pip install -r requirements.txt
 
 The first command ensures you are using an up to date version of `pip`,
 and the second command installs all dependencies.
-The third command ensures to install the BIDS schema code as an "editable" install,
-so that if you make changes to the schema files,
-these are automatically reflected in the sourcecode.
 
 Note that if you need to work on the some of the Python code
 that is used to render the specification,