[gdb/testsuite] Drop unnecessary -Wl,-soname in gdb.base/skip-solib.exp

[gdb/testsuite] Drop unnecessary -Wl,-soname in gdb.base/skip-solib.exp

I noticed in gdb.base/skip-solib.exp:
...
if {[gdb_compile_shlib ${srcdir}/${subdir}/${srcfile_lib} ${binfile_lib} \
        [list debug -Wl,-soname,${libname}.so]] != ""} {
    return -1
}
...
that the -Wl,-soname argument is missing an ldflags= prefix, but adding it
gives us a duplicate:
...
Executing on host: gcc -fno-stack-protector \
  outputs/gdb.base/skip-solib/skip-solib-lib.c.o  -fdiagnostics-color=never \
  -shared -g  -Wl,-soname,libskip-solib.so -Wl,-soname,libskip-solib.so -lm \
  -o outputs/gdb.base/skip-solib/libskip-solib.so    (timeout = 300)
...
so apparently it's taken care of by gdb_compile_shlib.

Drop the inactive and also unnecessary -Wl,-soname,${libname}.so from the
flags list for the gdb_compile_shlib call.

Tested on x86_64-linux.