Remove pointless information from README.md

Those who need to build the extension within the source tree, already
know how to do it.

README.md

@@ -8,10 +8,7 @@ not resizable, thus, no new plans can be added once it has been filled up.*
 
 # INSTALL
 
-Either use PGXS infrastructure (recommended), or compile within the source
-tree. PostgreSQL versions 12 and newer are supported.
-
-## PGXS
+PostgreSQL versions 12 and newer are supported.
 
 Install PostgreSQL before proceeding. Make sure to have `pg_config` binary,
 these are typically included in `-dev` and `-devel` packages.
@@ -23,22 +20,6 @@ make
 make install
 ```
 
-## Within Source Tree
-
-```bash
-PG_VER='15.3' # Set the required PostgreSQL version.
-curl -O "https://download.postgresql.org/pub/source/v${PG_VER}/postgresql-${PG_VER}.tar.bz2"
-tar xvfj "postgresql-${PG_VER}.tar.bz2"
-cd postgresql-${PG_VER}
-./configure
-
-cd contrib
-git clone https://github.com/cybertec-postgresql/pg_show_plans.git
-cd pg_show_plans
-make USE_PGXS=
-make USE_PGXS= install
-```
-
 ## Configure
 
 Add `pg_show_plans` to `shared_preload_libraries` within `postgresql.conf`: