patch 9.1.1923: wrong error when assigning to read-only register

patch 9.1.1923: wrong error when assigning to read-only register

Problem:  When assigning to @. in a :let command an incorrect "E15"
          error is emitted.
Solution: Emit the correct "E354" error. (Doug Kearns).

An incorrect "E488" error was also emitted in Vim9 script assignments.

It appears that the code deleted in this commit was added to work around
a limitation in the returned value from find_name_end() that no longer
exists.

See commit 76b92b2830841fd4e05006cc3cad1d8f0bc8101b (tag: v7.0b).

closes: #18757

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>