Drop "install" target and documentation.

This has hurt many many many people over it's lifetime, by confusing their
multilib toolchains.  Simply drop it outright.  People who _really_ know what
they're doing are still perfectly entitled to "install" portions of this
project in suitable locations for their own use.

README.md

@@ -134,13 +134,9 @@ https://github.com/libopencm3/libopencm3-examples
 Installation
 ------------
 
-    $ make install
-
-This will install the library into `/usr/local`. (permissions permitting)
-
-If you want to install it elsewhere, use the following syntax:
-
-    $ make DESTDIR=/opt/libopencm3 install
+Simply pass -I and -L flags to your own project.  See the libopencm3-examples
+repository for an example of using this library as a git submodule, the most
+popular method of use.
 
 It is strongly advised that you do not attempt to install this library to any
 path inside your toolchain itself.  While this means you don't have to include
@@ -149,7 +145,6 @@ linker from picking the right versions of libraries.  Common symptoms are
 hardfaults caused by branches into arm code.  You can use `arm-none-eabi-objdump`
 to check for this in your final elf.  You have been warned.
 
-
 Coding style and development guidelines
 ---------------------------------------