Remove libsoftcrypto.h under jdk.crypto.ucrypto once devkit is upgraded

After 8046002, OracleUcrypto provider is added to OpenJDK without the Solaris headers it depends on. This is due to the assumption that the minimum Solaris version (S11) for JDK 9 should already have the necessary header files. However, the official Solaris build is still done using S10u6 devkit which doesn't have the necessary Ucrypto headers.
As a temporary work around, we added a custom libsoftcrypto.h. Once devkit is upgraded to a more recent version, this custom libsoftcrypto.h should be removed.