Don't assume epkg is available

[thblt]

I'm not sure this is the correct solution, but M-x borg-build has been failing recently because I don't have epkg installed. This commit modifies borg-elpa-initialize and package-load-descriptor--borg-use-database so they don't assume epkg is installed.

Here's an example of the aforementioned error:

(17:51:28) Building borg

Package autoload is deprecated
Cannot open load file: No such file or directory, epkg

Error: file-missing ("Cannot open load file" "No such file or directory" "epkg")
  debug-early-backtrace()
  debug-early(error (file-missing "Cannot open load file" "No such file or directory" "epkg"))
  require(epkg)
  (let ((load-path (nconc (cl-mapcan #'(lambda (name) (let ((dir (expand-file-name name borg-drones-directory))) (if (file-directory-p dir) (list dir) nil))) '("emacsql" "closql" "epkg")) load-path))) (require 'epkg))
  (if (featurep 'epkg) nil (let ((load-path (nconc (cl-mapcan #'(lambda (name) (let ((dir (expand-file-name name borg-drones-directory))) (if (file-directory-p dir) (list dir) nil))) '("emacsql" "closql" "epkg")) load-path))) (require 'epkg)))
  borg-elpa-initialize()
  (progn (setq user-emacs-directory "/home/thblt/.emacs.d/") (require 'package) (package-initialize 'no-activate) (package-activate 'borg) (require 'borg-elpa) (borg-elpa-initialize) (setq borg-build-shell-command 'nil) (borg-build "borg"))
  command-line-1(("-L" "/home/thblt/.emacs.d/lib/borg/" "--eval" "(progn\n  (setq user-emacs-directory \"/home/thblt/.emacs.d/\")\n  (require 'package)\n  (package-initialize 'no-activate)\n  (package-activate 'borg)\n  (require 'borg-elpa)\n  (borg-elpa-initialize)\n  (setq borg-build-shell-command (quote nil))\n  (borg-build \"borg\"))"))
  command-line()
  normal-top-level()

Process emacs ... --eval (borg-build "borg") exited abnormally with code 255

Thanks!