fix(build & doc): Remove remaining mentions of ldpc_cuda

* Option to build ldpc_cuda was remaining in the build script
* Build script option to build ldpc_cuda was still used in CI
* Mentions of ldpc_cuda were remaining in the documentation

Note: In the documentation, the example of ldpctest usage that was
loading ldpc_cuda was replaced by an example in which it is loading
ldpc_orig even though this is pointless as it makes ldpctest compare
ldpc_orig with ldpc_orig. On the other hand, it is the only library
other than the default ldpc that uses the former interface.

openair1/PHY/CODING/DOC/LDPCImplementation.md

@@ -110,10 +110,10 @@ A mechanism to select ldpc implementation is also available in the `ldpctest` ph
 
 #### Examples of ldpc shared lib selection when running ldpctest:
 
-Loading libldpc_cuda.so, the cuda implementation of the ldpc decoder:
+Loading libldpc_orig.so, the legacy implementation of the ldpc decoder:
 
 ```
-$ ./ldpctest -v _cuda
+$ ./ldpctest -v _orig
 ```
 
 ### LDPC libraries
@@ -124,6 +124,4 @@ Libraries implementing the LDPC algorithms must be named `libldpc<_version>.so`,
 * `LDPCdecoder` implementing type `LDPC_decoderfunc_t *`
 * `LDPCencoder` implementing type `LDPC_encoderfunc_t *`
 
-`libldpc_cuda.so` has been tested with the `ldpctest` executable, usage from the softmodem's has to be tested.
-
 [oai Wikis home](https://gitlab.eurecom.fr/oai/openairinterface5g/wikis/home)