Remove getSpecVersion() from Project interface

With the latest changes in SAP/ui5-project#431,
the method now returns an instance of a "SpecificationVersion" class.
This would require an additional specVersion-dependent interface.

Since use cases in custom tasks and -middelare are not clear, we should
rather keep it simple for now and do not provide this method.

rfcs/0012-UI5-Tooling-Extension-API-3.md

@@ -144,7 +144,6 @@ UI5 Tooling v3 has switched the internal project handling from a JSON-hierarchy
 
 `Project` interface for Specification Version 3:
 
-* `getSpecVersion`: Returns the specification version of the project. E.g. "3.0"
 * `getType`: Returns the type of the project. E.g. "library", "application", etc.
 * `getName`: Returns the name of the project
 * `getNamespace`: Returns the detected namespace of the project. E.g. `x/y/z`