[gdb/testsuite] Drop -readnow in three gdb.dwarf2 test-cases

[gdb/testsuite] Drop -readnow in three gdb.dwarf2 test-cases

When running the testsuite in an enviroment simulating a stressed system, I
ran into timeouts in three test-cases in gdb.dwarf2:
- gdb.dwarf2/count.exp,
- gdb.dwarf2/implptrconst.exp, and
- gdb.dwarf2/implptrpiece.exp.

In all three cases, -readnow is used which results in symtabs being expanded for
the executable, /lib64/libc.so.6 and /lib64/ld-linux-x86-64.so.2.

We could address this by limiting the scope of -readnow to the executable, but
after reviewing the test-cases there doesn't seem to be a clear reason to use
-readnow.

Fix this by dropping the -readnow.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>