Don't use equalp at run-time, compare downcased strings

Don't use equalp at run-time, compare downcased strings

* org-bibtex.el (org-bibtex-headline): Don't use equalp at run-time,
compare downcased strings.

Compiler complains about using `equalp' because other than for example
`concatenate' it is not declared 'inline.

Cf. [[info:cl#Declarations]] about the 'inline declaration

...
`inline'
     The `inline' DECL-SPEC lists one or more functions whose bodies
     should be expanded "in-line" into calling functions whenever the
     compiler is able to arrange for it.  For example, the Common Lisp
     function `cadr' is declared `inline' by this package so that the
     form `(cadr X)' will expand directly into `(car (cdr X))' when it
     is called in user functions, for a savings of one (relatively
     expensive) function call.

     The following declarations are all equivalent.  Note that the
     `defsubst' form is a convenient way to define a function and
     declare it inline all at once.

          (declaim (inline foo bar))
          (eval-when (compile load eval) (proclaim '(inline foo bar)))
          (defsubst foo (...) ...)       ; instead of defun

     *Please note:*  this declaration remains in effect after the
     containing source file is done.  It is correct to use it to
     request that a function you have defined should be inlined, but it
     is impolite to use it to request inlining of an external function.

     In Common Lisp, it is possible to use `(declare (inline ...))'
     before a particular call to a function to cause just that call to
     be inlined; the current byte compilers provide no way to implement
     this, so `(declare (inline ...))' is currently ignored by this
     package.
...

Thus other than `concatenate' `equalp' is stored as a function call at
run-time.