Remove "\nError: " suffix from nat/fork-inferior.c:trace_start_error warning message

Remove "\nError: " suffix from nat/fork-inferior.c:trace_start_error warning message

Rationale: https://sourceware.org/ml/gdb-patches/2019-08/msg00651.html

This very simple patch removes the "\nError: " suffix from the warning
message printed by nat/fork-inferior.c:trace_start_error.  This proved
to just pollute the screen, causing things like:

  Starting program: /usr/bin/true
  warning: Could not trace the inferior process.
  Error:
  warning: ptrace: Permission denied

This "Error: " string is not useful at all, and can confuse things,
therefore let's just remove it and simplify the resulting messages:

  Starting program: /usr/bin/true
  warning: Could not trace the inferior process.
  warning: ptrace: Permission denied

gdb/ChangeLog:
2019-08-30  Sergio Durigan Junior  <sergiodj@redhat.com>

	* nat/fork-inferior.c (trace_start_error): Remove "\nError: "
	suffix from warning message.