Avoid touching source files in profile-build

Avoid touching source files in profile-build

This refines the profile-build target to avoid 'touch'ing the sources,
keeping meaningful modification dates and avoiding editor warnings like vi's:

WARNING: The file has been changed since reading it!!!
Do you really want to write to it (y/n)?

Instead of touching sources, the (instrumented) object files are removed,
which has the same effect of rebuilding them in the next step.

As a side effect, this simplifies the Makefile a bit.

No functional change.