mainpage.dox: Remove reference to LREADME.

This reference no longer exists and was not particularly useful either.

Before this fix, following warning was generated during 'make':
avm/mainpage.dox:20: warning: unable to resolve reference to 'LREADME'
for \ref command

mainpage.dox

diff --git a/mainpage.dox b/mainpage.dox
index 10924ac..3ec26e6 100644
--- a/mainpage.dox
+++ b/mainpage.dox
@@ -21,11 +21,7 @@
   \subsection main_startpoints Starting Points
   - Consult the \ref changelog for a complete list of improvements in this
     release.
-  \if av1_md_support
-  - [README](\ref LREADME) contains instructions on compiling the sample applications.
-  \else
   - \ref readme contains instructions on compiling the sample applications.
-  \endif
   - Read the \ref usage "usage" for a narrative on codec usage.
   - Read the \ref samples "sample code" for examples of how to interact with the
     codec.