Revert "Change OTLP/HTTP port from 4317 to 4318 (#1839)" (#1847)

Apparently there is new evidence that one port may work after all:
https://github.com/open-telemetry/opentelemetry-collector/pull/3765/files

This reverts commit cce1d5996873de38a68e05eafa4d5e224df9b8f1 until we have the final
decision about the ability to have a single port, see:

open-telemetry/opentelemetry-specification#1846 (comment)

specification/protocol/otlp.md

@@ -473,7 +473,7 @@ connections SHOULD be configurable.
 
 #### OTLP/HTTP Default Port
 
-The default network port for OTLP/HTTP is 4318.
+The default network port for OTLP/HTTP is 4317.
 
 ## Implementation Recommendations