The full text of the license is here.
Section 2.3 can only be understood in context with Section 2.1 b. 2.1 b is a general statement that a license is granted (i.e. the contributor may not charge any royalties for his/her contributions nor for the Contributor Version to which he/she contributed), while 2.3 limits the scope of this license.
The part of the Contributor Version is critical here, because it means that you grant a patent license even for functionality earlier contributed by others as soon as you contribute something (even something minor or unrelated to the patented functionality) to the project.
2.3a covers the case that a Contributor removes code (you could call it a 'negative contribution'). From the license context it is clear that this refers to code of another contributor, because if that code was part of the Contributor's own contribution the license for that functionality would already be in place since the time of that earlier contribution.
So Section 2.3a just clarifies the license situation in case of a 'negative contribution'. This could be important in the case when a Contributor actually owns a patent right (for functionality x) which they don't want to license for free, and where someone else included code for functionality x into the project. If then the Contributor wants to contribute some code for functionality y (unrelated to functionality x), then the Contributor might want to remove code related to functionality x from the Contributor Version in order not to be forced to license their patent.