[gdb/doc] Remove trailing done from "Reading symbols from" lines

[gdb/doc] Remove trailing done from "Reading symbols from" lines

Since commit 3453e7e409 'Clean up "Reading symbols" output' we no longer print
"done." after the "Reading symbols from" message:
...
$ gdb -q a.out
Reading symbols from a.out...
(gdb)
...

Update docs accordingly.

Build on x86_64-linux.

gdb/doc/ChangeLog:

2020-03-04  Tom de Vries  <tdevries@suse.de>

	* gdb.texinfo: Remove trailing "done." in "Reading symbols from" lines.
	* python.texi: Same.