Update author info.

Update author info.

I had put Jia Tan in RISC-V authors in XZ Embedded largely as a courtesy.
The adaptation of the RISC-V decoder to XZ Embedded I did alone based
on the liblzma code in XZ Utils. Jia had only done minor work on the
RISC-V filter *code* in liblzma, adding macros to avoid repeated code
and making improvements to comments, which I reviewed and edited further.
The initial commit in XZ Utils (440a2eccb082dc13400c09e22308a58fef85146c)
was made by Jia but the riscv.c file in that commit was almost solely
written by me (and matched the file I had outside of the Git tree).

Thus it's fine to remove Jia's name from XZ Embedded author info.