Yes, there is a way to remove it.
I quickly hacked together a simple addon that does this via an XUL Overlay specified in its chrome.manifest. I don't see a good way to achieve this without an addon or modifying files.
Technical details:
The original file, chrome/messenger/content/messenger/quickFilterBar.xul
located inside omni.ja
sets the text with the following XUL tag using the attribute emptytextbase
(on line 139, for Thunderbird 24.5.0):
<textbox id="qfb-qs-textbox" flex="1"
type="search"
placeholder=""
emptytextbase="&quickFilterBar.textbox.emptyText.base;"
...
Just for reference: &quickFilterBar.textbox.emptyText.base;
is defined in chrome/<lang>/locale/<lang>/messenger/quickFilterBar.dtd
, also located inside omni.ja
.
The XUL Overlay inside my addon changes the text to an empty string with the following line:
<textbox id="qfb-qs-textbox" emptytextbase=""></textbox>