README: Remove old note about the build system's python expectation

Changesets 5852ca4852 (build: fix tools/configure in case only python3
exists) and c8a8645f1e ("xen/build: Automatically locate a suitable python
interpreter") cause the build to look for python3, python and python2 in that
order.

Remove the outdated note from the README.

Signed-off-by: Javi Merino <javi.merino@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

README

@@ -184,13 +184,6 @@ Various tools, such as pygrub, have the following runtime dependencies:
           URL:    https://www.python.org/
           Debian: python
 
-Note that the build system expects `python` to be available. If your system
-only has `python2` or `python3` but not `python` (as in Linux From Scratch),
-you will need to create a symlink for it, or specify PYTHON= when invoking
-make, like (note the position of PYTHON= matters):
-
-    # make PYTHON=/usr/bin/python3
-
 Intel(R) Trusted Execution Technology Support
 =============================================