remove redundant method calls in DatatypeConverterImpl

[winfriedgerlach]

• both new BigInteger() and Long.parseLong() accept strings with + prefix, there is no need to remove anything (maybe the confusion came from an error in the JavaDoc, I noticed that in Java 11 the JavaDoc for BigInteger(String) doesn't mention the + sign while the JavaDoc for BigInteger(String, int) is correct; the JavaDoc was fixed in later Java versions)
• the extra checks for . or non-digits also make no sense, because they will lead to a NumberFormatException anyway
• there should be a slight performance benefit as well, but microbenchmarking showed that this is insignificant --> correct semantics are more important here

[laurentschoelens]

I'd be happy to see every use case handled in the removed function in order to check if nothing is broken.

Also please update copyrights header regarding Oracle or Eclipse situation you're in